Process for coloring aluminum objects

ABSTRACT

PROCESS FOR PRODUCING COLORED PROTECTIVE COATINGS ON OBJECTS OF ALUMINUM OR ALLOYS OF ALUMINUM IN WHICH THE SURFACE IS PROVIDED WITH A POROUS LAYER OF ALUMINA AND IN WHICH METAL PIGMENTS ARE DISPOSITED INTO THE PORES OF THE LAYER BY ALTERNATING CURRENT ELECTROLYSIS WITH THE OBJECT MOUNTED AS AN ELECTRODE IN AN AQUEOUS ACID SOLUTION OF METAL SALTS CONTAINING NICKEL SULPHATE AND NICKEL CHLORIDE AS ESSENTIAL SALTS.

United States Patent 3,704,210 PROCESS FOR COLORING ALUMINUM OBJECTS Jos Pattie, Grenoble, France, assignor to Cegedur GP No Drawing. Filed July 15, 1970, Ser. No. 55,295 Claims priority, application France, July 21, 1969, 6924728 Int. Cl. C2315 7/06 US. Cl. 20435 N 2 Claims ABSTRACT OF THE DISCLOSURE Process for producing colored protective coatings on objects of aluminum or alloys of aluminum in which the surface is provided with a porous layer of alumina and in which metal pigments are deposited into the pores of the layer by alternating current electrolysis with the object mounted as an electrode in an aqueous acid solution of metal salts containing nickel sulphate and nickel chloride as essential salts.

This invention relates to colored protective surface layers on aluminum or alloys of aluminum and methods and compositions for the preparation of same.

Such coatings have been produced in various ways, one of which consists of forming a porous surface on the aluminum object by anodization, depositing metal pigments into the pores by alternating current electrolysis in an acid solution of metal salts with the object itself as one of the electrodes while the other electrode or counterelectrode is formed of stainless steel or more often of the metal of the salt dissolved in the electrolyte, and then sealing the colored layer by immersion in boiling water.

By this technique, it is possible theoretically to obtain shades of bronze, when use is made of an electrolyte containing nickel sulphate and a counter-electrode formed of nickel. Thus a pure aluminum sheet of 99.9% aluminum, previously treated to provide a satin finish and anodized in a sulphuric acid bath to provide an anodized oxide layer having a thickness of 20 microns, can be colored bronze by mounting the sheet as an electrode in a bath containing 100 g./liter of nickel sulphate, 30 g./ liter of ammonium sulphate and 25 g./liter of boric acid, with the bath adjusted to a pH of 4.7 and with a resistivity of 25.1 ohms cm. /cm. The treatment is carried out at 25 C. with an effective amps/dm. of 0.4 for a period of 3 minutes with a counter-electrode of nickel and with a ratio of surface area of the sheet to surface area of the counter-electrode of 2.

After scaling in boiling distilled water for 45 minutes, a bronze shade is obtained which is resistant to corrosion and ultra-violet rays.

However, during the coloring operation in alternating current, it has been found that the nickel counterelectrode becomes covered with a blackish deposit and that the electrolysis voltage increases from to 11.1 volts. This indicates that the counter-electrode has been polarized. This is a very undesirable phenomenon which makes it difficult commercially to color large surfaces with a homogeneous dark bronze shade.

Furthermore, crystallization occurs at ambient temperature and, upon standing, the crystals fall to the bottom of the bath. As a result, when electrolysis is removed, a time lapse is required to bring the bath up to temperature sufficient to re-dissolve all of the components of the bath.

It is an object of this invention to produce colored aluminum objects having improved uniformity and intensity of color from batch to batch, and which makes use of an improved method and composition for use in Patented Nov. 28, 1972 the preparation of same, which is not subject to the deficiencies heretofore pointed out, which is stable during periods of use and non-use, and which is adapted for the production of colored aluminum surfaces on a commercial scale.

In accordance with the practice of this invention, the electrolysis bath is formulated to contain dissolved nickel chloride as a component thereof whereby crystallization in the electrolyte does not occur at ambient temperature and polarization of the counter-electrode does not take place. As a result, it becomes possible to obtain homogeneous dark shades over surfaces of large surface area and moreover the system is characterized by markedly improved electrical conductivity with corresponding savings in time and energy.

Generally, the process applies to aluminum and its alloys. The object can be subjected to a surface treatment beforehand, depending upon the final appearance required. For example, a mat or dull appearance is obtained by pickling the object for 10 minutes at 50 C. in a 50 g./liter soda solution. The object is rinsed, immersed in nitric acid at 36 Be and then re-rinsed. A bright appearance is obtained either by mechanical polishing or by chemical or electrolytic brightening. A porous layer of alumina 1 to 50 microns thick is then formed on the object by any known means, for example anodization in a 50 g./ liter sulphuric or chromic acid bath using direct or alternating current. This layer is then colored using alternating current. In the process according to the invention, the operating conditions are as follows:

Broad Narrow Composition of the electrolyte:

Nickels phate l to 200 g./l 5 to 100 g./l. Ammonium sulphate. 1 to 100 g 10 to 50 g./1. Boric acid 1 to 100 g 0 gJl. Nickel chloride 1 to 500 g pH value Temperature.

C0unter-electrode Electrical conditions Alternating current; Either at a constant density of from 0.1 to 5 A. etildmfl, preferably 0.1 to 1 A.

efiJdmfl; or at a constant voltage of from 5 to v., preferably 7 to 30 v.; duration from 5 seconds to 20 minutes, preferably 1 to 10 minutes.

A sheet of 99.5% pure aluminum which has been chemically brightened and anodized to a layer thickness of 10 microns in a chromic acid bath is colored by treatment with alternating current in a bath containing: g./liter of nickel sulphate, 10 g./1iter of ammonium sulphate, 20 g./liter of boric acid, 30 g./liter of nickel chloride, with a pH value of 4.55, a resistivity of 21.5 ohms cmP/cm. and with crystals at the bottom of the cell, at 20 C.

Treatment was carried out with a voltage of 10 volts at a temperature of 22 C. for a period of 5 minutes with a ratio of surface area of component to surface area of counter-electrode of 3.

The stainless steel counter-electrodes retain a clean appearance and the current density varies from 0.35 to 0.4 a. eif/dm. during the treatment.

It is possible in this way to obtain a fairly deep bronze color finish which is resistant both to corrosion and to ultra-violet rays.

3 EXAMPLE 2 A sheet of aluminum alloy containing 0.6% of magnesium, which has been electrolytically brightened and anodized to a layer thickness of microns in a sulphuric acid bath, is colored by treatment with alternating current in a bath containing: 100 g./liter of nickel sulphate, 30 g./liter of ammonium sulphate, 25 g./liter of boric acid and 200 g./liter of nickel chloride, with a pH value of 3.99 and a resistivity of 15.2 ohms cmF/cm. At C. there are no crystals at the bottom of the cell.

The treatment was carried out at C. at a current density of 0.4 a. eif./dm. for a period of three minutes with a ratio of surface area of component to surface area of counter-electrode of 2.

The nickel counter-electrodes retain a clear appearance and the electrolysis voltage varies from 9 to 9.6 volts during the treatment. It is possible in this Way to obtain a bronze color finish which, after minutes sealing in boiling distilled water, containing 1 g./1iter of nickel acetate, is resistant both to corrosion and to ultra-violet rays.

EXAMPLE 3 A piece of aluminum alloy section containing 0.5% of silicon and 0.5 of magnesium, which has been mechanically polished and anodized to a layer thickness of 12 microns in a sulphuric acid bath, is colored by treatment with alternating current in a bath containing: 100 g./liter of nickel sulphate, 30 g./liter of ammonium sulphate, 25 g./liter of boric acid and 200 g./liter of nickel chloride, with a pH value of 3.99 and a resistivity of 15.2 ohms cmF/cm. At 20 C. there are no crystals at the bottom of the cell.

The treatment was carried out at 30 C. at a current density of 0.4 a. e1T./dm. for a period of 10 minutes with a ratio of surface area of component to surface area of counter-electrode of 2.

The nickel counter-electrodes retain a clear appearance and the electrolysis voltage varies from 9 to 9.7 volts during the treatment. It is possible in this way to obtain a very deep bronze color finish which, after 30 minutes sealing in boiling distilled water, is resistant both to corrosion and to ultra-violet rays.

EXAMPLE 4 A piece of a section of aluminum alloy containing 1% of silicon, 1% of magnesium and 1% of manganese, which has been satinized and anodized to a layer thickness of 5 microns in an alternating current sulphuric acid bath, is colored by treatment with alternating current in a bath containing: 75 g./liter of nickel sulphate, 20 g./liter of ammonium sulphate, g./liter of boric acid and 350 g./liter of nickel chloride, with a pH value of 3.6 and a resistivity of 14.8 ohms cmF/cm. At 20 C., there are no crystals at the bottom of the cell.

The treatment was carried out for 10 minutes at 22 C. with a current density of 0.4 a. elf./dm. and a ratio of surface area of component to surface area of counterelectrode of 4.

The stainless steel counter-electrodes retain a clean appearance and the electrolysis voltage remains constant at 9.6 volts, during the treatment. It is possible in this way to obtain a very deep bronze color finish which, after 30 minutes sealing in boiling distilled water, is resistant both to corrosion and to ultra-violet rays.

The preceding examples show that the addition of nickel chloride enables:

polarization of the counter-electrodes to be avoided;

crystallization of the electrolyte at room temperature to be reduced and even entirely avoided when the nickel chloride content is greater than 30 g./liter, and

resistivity of the electrolyte to be reduced to It will be apparent from the foregoing that this invention provides a new and improved electrolysis bath for the production of uniform bronze colors on aluminum and alloys of aluminum in a simple, efficient and reproducible manner.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. A process for producing colored surfaces of aluminum and alloys of aluminum comprising contacting an anodized surface of aluminum or alloy of aluminum as one of the electrodes with an electrolysis bath comprising an acidic solution of 5 to grams per liter hydrated nickel sulfate, 10 to 50 grams per liter ammonium sulfate and 50 to 250 grams per liter hydrated nickel chloride, and passing an alternating current between the electrodes.

2. A process as claimed in claim 1 which includes the step of sealing the colored surface by immersion in boiling water.

References Cited UNITED STATES PATENTS 3,382,160 5/1968 Asada 204-35 N FOREIGN PATENTS 762,911 7/ 1967 Canada 204-35 N OTHER REFERENCES Weast, Robert C.: Handbook of Chemistry and Physics, The Chemical Rubber Co., 45th ed., pp. 3-197 to 13-198.

JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. Cl. X.R. 20442, 58 

